1. Field of the Invention
The present invention relates to an ultrasonic imaging apparatus configured to transmit and receive ultrasonic waves to and from a living body to obtain medical image data and display a three-dimensional image, and more specifically, relates to an ultrasonic imaging apparatus configured to display a three-dimensional color Doppler image of blood-flow information.
2. Description of the Related Art
In medical image diagnosis using an ultrasonic imaging apparatus, an X-ray CT apparatus, an MRI apparatus etc., in accordance with increase of the speed and performance of the detecting function and arithmetic process of biological information, it has become possible to perform real-time display of displaying an image almost simultaneously with acquisition of image data from a subject.
In each of the image diagnosis apparatuses described above, a plurality of different physical quantities indicating the biological characteristics of the subject are detected and imaged. An ultrasonic imaging apparatus has a plurality of capturing modes, such as the B-mode method of imaging tissue structures within a living body by using the magnitude of reflected waves of ultrasonic waves having been transmitted to the subject (hereinafter referred to as “ultrasonic echoes”), and the color Doppler method of imaging a blood flow or an organ movement velocity by using the ultrasonic Doppler effect.
Information obtained from the ultrasonic echoes in this color Doppler method is blood-flow-velocity information (its sign varies between the case of getting away from the ultrasonic probe and the case of coming close to the ultrasonic probe), power information in which velocity is differentiated (power information indicates the volume of a blood flow per unit time), and dispersion information in which variations in velocity is presented as an index.
The color Doppler method includes an image display mode such as a velocity and dispersion display mode of displaying velocity information and dispersion information of a blood flow, and a power display mode of displaying power information. This makes it possible to simultaneously display the velocity information, power information, dispersion information of a blood flow, etc. by displaying blood-flow information converted into a color map, in an ultrasonic imaging apparatus generating a two-dimensional image. For example, by using two physical quantities as parameters, arranging the parameters on the vertical axis and the horizontal axis of a two-dimensional map, respectively, and creating a color map corresponding to respective values thereof, a doctor and a laboratory technician (hereinafter referred to as an “operator”) can identify the conditions of the two change quantities with colors.
Further, in recent years, as an ultrasonic imaging apparatuses, a system capable of high-speed collection and display of three-dimensional images has been rapidly developed. Thus, it has become possible to provide a diagnostic image beyond a conventional visual field, such as a three-dimensional image and a moving image of a three-dimensional image.
Furthermore, a three-dimensional ultrasonic imaging apparatus has become capable of not only forming a three-dimensional image of a living-body histological image (i.e., an image produced by the B-mode method) but also displaying a three-dimensional image by combining a three-dimensional image of an image produced by the color Doppler method (a color Doppler image) and a histological image. In this regard, in generation of a three-dimensional image, it is common to generate a three-dimensional image of a histological image, and it is uncommon to generate images of other physical parameters. However, since a color Doppler image provides a visualized image of information on a blood flow in a body, it is possible to display three-dimensional information on a blood flow by displaying as a three-dimensional image, which is becoming important display.
In the generation of a three-dimensional image, volume rendering is used. This volume rendering is as described below. For example, slice images obtained by an ultrasonic imaging apparatus are stacked. Next, a volume model having a three-dimensional structure (a voxel space) is created in which the value of each of the plurality of slice images is put into a square called a voxel. After the angle of view is determined for this volume model, voxel tracking is executed from an arbitrary viewpoint, and transmitted light and reflected light are obtained based on the opacity corresponding to the voxel value, whereby brightness is obtained. Moreover, image information based on this brightness is projected on pixels of a projection plane, and organs are sterically extracted, whereby a three-dimensional image is obtained.
Here, in the ultrasonic imaging apparatus generating and displaying a three-dimensional color Doppler image, it is preferred to three-dimensionally display the aforementioned parameters including the velocity information, power information and dispersion information of a blood flow. In this regard, as in the case of a two-dimensional image, it is possible to create a 3D color map in which colors corresponding to the three parameters are set, respectively. However, for a reason such that the human cognitive ability does not correspond, it is necessary to use a color map corresponding to two parameters in the case of displaying the values of the parameters in colors.
In general, in the case of displaying a three-dimensional image, it is necessary to project and display on a two-dimensional projection plane so that a human can easily perceive three-dimensionally arranged values. That is, the greatest difference between display of a three-dimensional image and display of a two-dimensional image is that depth information must be visualized. Moreover, in the case of a three-dimensional medical image, it is often necessary to visualize three-dimensional image data of not only the surface but also the inside.
In this regard, in three-dimensional imaging of tissue information, data equivalent to a 2D luminance signal is allocated to voxel data. Then, it is possible not to display a portion with luminance lower than a certain value by setting a threshold and transparency in accordance with the luminance, and it is possible to three-dimensionally depict a tissue with high luminance by setting higher level of luminance to be more opaque. In the case of applying this to the color Doppler method, for example, it is possible to set transparency in accordance with the velocity value, and it is possible to three-dimensionally image only a high-velocity blood flow. However, in the case of a two-dimensional color Doppler image, as described above, two parameters (e.g., the velocity value and dispersion value, or the velocity value and power value) may be simultaneously displayed in color. In this case, it is difficult to convert a 2D display image into a three-dimensional image as it is.
Thus, a technique has been proposed conventionally in which setting of opacity and setting of color tone are performed based on values of different parameters when a three-dimensional image is generated based on ultrasonic echoes obtained from a subject body by the color Doppler method (for example, refer to Japanese Unexamined Patent Application Publication JP-A 2005-143733.)
However, the conventional ultrasonic apparatus gives one physical quantity as a parameter into one voxel data and generates a three-dimensional image based thereon. Then, the apparatus performs an operation of stacking three-dimensional images generated with different parameters. Therefore, in the conventional ultrasonic apparatus, calculation for generating a three-dimensional image is complicated and processing is heavy, and moreover, effective use of the respective parameters is difficult.
In addition, in a case where a parameter indicating one physical quantity is assigned to each of the opacity setting and the color tone setting as in JP-A 2005-143733, each of the physical quantities is handled separately.
Therefore, it is impossible to use a part of allowing the operator to easily recognize the state of the subject by associating the two parameters and displaying the values in one color. Accordingly, it is difficult to reflect the relationship between the two parameters. In addition, it is difficult to display only a part conforming to the condition of the values of the two parameters.
Therefore, it is difficult to effectively display a three-dimensional image.